Matt
couple of thoughts here:
1. LEDS:
those leds you mention are the LEDs that are run on 20mA (DC foraward current) and thus NOT the brightest on the planet.
These leds
http://www.philipslumileds.com/uploads/3/DS05-pdf
are Philips Superflux and have a forward current of 70mA. that makes a big difference.
2. COLOR:
your leds have a wavelength of 640nm...that is RED...behind your red lens of the taillight a lot of the output will be lost beause of that.
the leds I used (and that are used by allmost all on hidplanet.com) are of the orange-red kind that have a wavelength of 626nm.....difference seems little, but in output makes a hell of a difference:
I have bought 6 different LEDS to test these things and found out myself. And the guys from the hidplanet.com forum had told me in their first comment to go with the Lumileds.
resistor calculation:
keep in mind that you are not feeding the LEDS 12V, but 11.3V because there should be a diode inline after the voltage regulator.
as far as voltage regulator. why use an adjustable? to control brightness?
brightness isnt only partially controllable by voltage...under a certain voltage they just turn off...that is why some guys build the PWM circuits for dimming teh arrays. I just did some testing behind the lens with several resistors and found the correct size for my application.
keep in mind to allways check if the wattage of the resistor is big enough to dissipate the heat it will build up.
just some thoughts....but go for the Lumileds Orange-Red...you wont regret it...they are better quality, last longer and blind the **** out of the cars behind you when you hit the brakes...hahahaha